Honestly, for sanity's sake, I would just call your PO and give them the tracking # and ask them to look it up. Their system will offer more details than the consumer-facing website.

In general, this just means delivery couldn't be completed. But specifically, it could mean anything from "the mailman is hoarding undelivered mail in his attic" to "the machine ate it" to "the item could not be securely delivered so we didn't leave it." Rather than wonder/worry which it is, you're better off just asking if the PO can provide a real answer!

I've had this happen a couple of times with the mail going from the main Denver facility to my Post Office and it seems to happen at the main facility. When I called my Post Office, they hadn't received it yet. Seems to hang in limbo for a couple of days to about a week, but has always shown up eventually. Never got a good explanation what it really meant.

Yeah, I've gotten it whenever the target date isn't met. As others said, it just means something happened and it won't be arriving when planned. So far, for me, they always arrive eventually. KNOCK WOOD!!!